The development of more powerful and sophisticated electronic equipment, such as that comprising a computer system, involves an ever increasing circuit package density. The combination of smaller physical size and increased power requirements results in complex multilayer printed circuit boards or cards. Concomitant with such complexity, there exists the increased probability of defects. Although most defects present in the cards themselves, or in the components disposed thereon, are detected at an early stage by routine inspection procedures, some latent defects escape detection. Therefore, it is possible for defective cards to be assembled and installed in the computer system.
After the system has been operating for some time, the aforementioned latent defects may surface, often with serious consequences. For example, a short circuit may develop between power planes in the respective layers of a card. The large amount of current flowing in such planes as a result of the short circuit causes a tremendous amount of heat to be generated within a small area. Since it is common for computer systems to operate for many hours at a site which is untended, the heat generated by the short circuit could destroy not only the card, but unless power is quickly removed from the equipment, could pose a threat of fire to the entire system and to the surrounding structures.
In practice, a large number of cards are mounted in close, parallel, spaced-apart relationship in a card rack disposed in a cabinet. The rack is air cooled by a blower located at the bottom of the cabinet. High velocity air is directed between the cards and exits the upper part of the cabinet. Although the cards are formed of material which will not support combustion and hence, are self extinguishing, heat from a short circuit, either within the card or in the components, will generate a small quantity of smoke. The latter is difficult to detect by a conventional smoke detector unit mounted in the top of the cabinet because of the card configuration and the dissipation of the smoke by the large volume of cooling air exiting the cabinet.
What is needed is an air sampling system which is highly responsive to smoke generated by short circuits in the cards and components or in the backplane wiring which interconnects the cards. Such a system must be reliable and effective at an early stage of the malfunction, and be capable of automatically shutting off electrical power to the card rack. Additionally, from a mechanical standpoint, the system should be universal. That is, the system components should be capable of being installed with little if any modification, in cabinets of various sizes and internal configurations. The smoke detection system of the present invention meets the foregoing requirements.